An understanding of the basic structure of chromatin as well as its modification is a central issue in biology. Chromatin structure inhibits many essential nuclear processes. Thus, precise control of chromatin structural modification is essential for many crucial cellular processes, including cell growth, development, and cell cycle progression. An understanding of the mechanisms controlling these events is fundamental to our understanding of human development and human diseases such as cancer. Access to DNA assembled into chromatin is facilitated by a growing family of ATP-dependent molecular machines. In the budding yeast, Saccharomyces cerevisiae, two related protein complexes (SWI/SNF and RSC) coordinate access to DNA. The SWI/SNF complex is essential for expression of a subset of yeast genes essential for mating-type switching and growth on alternative carbon sources. The complex rotationally destabilizes DNA sequences packaged into nucleosomes, facilitating recognition of DNA by transcription factors and other DNA binding proteins. The mechanism by which SWI/SNF recognizes target loci remains to be elucidated. Histone tails appear to play an important role in the modulation of catalytic SWI/SNF activity in vitro by controlling complex release from a targeted nucleosomal array. Trypsinization of arrays, removing histone tails, prevents the release of the SWI/SNF complex and remodeling of a second array. Acetylation of histone tails partially mimics this phenotype, suggesting that acetylation might enhance binding of SWI/SNF. The isolation of GCN5, a yeast histone acetyltransferase, as a SWI gene suggests that GCN5 and SWI/SNF function in a coordinated manner. These results suggest a model whereby GCN5 acetylation of histone tails facilitates SWI/SNF recognition and remodeling of particular nuclear loci. This project will assess the effects of individual histone tail deletions on SWI/SNF remodeling. Previous studies were performed with trypsinized arrays containing all four tailless histones. These studies will employ nucleosomal arrays produced from recombinant histones lacking only one histone tail. In addition to the remodeling studies themselves, the structural properties of these arrays are also of considerable interest. Trypsinized arrays lacking histone tails display aberrant folding in vitro. However, no studies have addressed the role of individual tails in these folding dynamics. Similar studies will be performed using in vitro GCN5 acetylated histones. In addition, we propose to identify SWI/SNF subunits involved in protein-protein contacts with histone tails by the incorporation of a protein crosslinking reagent into specific tails.